A frame is a structural component of a vehicle, such as an automobile or an aircraft, that uses a discrete, separate structure within a larger body-on-frame or unit body to carry certain components, such as the engine, drivetrain, or suspension (collectively referred to as a subassembly). The frame/subassembly is generally bolted and/or welded to the vehicle as a complete unit, which greatly simplifies the final vehicle assembly process. When bolted, it is sometimes equipped with rubber bushings or springs to dampen vibration.
One purpose of using a suspension subassembly is to isolate vibration and harshness from the rest of the body. For example, in an automobile with its powertrain contained in a suspension subassembly, forces generated by the engine and transmission can be damped enough that they will not disturb passengers. As a natural development from a car with a full chassis, separate front and rear suspension subassemblies are used in modern vehicles to reduce the overall weight, cost, and assembly complexity.
In certain situations, it may be desirable to include a steering actuator with the rear suspension subassembly to allow the rear wheels to be actively steered. In this manner, the vehicle may intelligently understeer/oversteer to promote overall stability and maneuverability.